lateral load resisting systems for wood structures (des130_awc08)

8/11/2019 Lateral Load Resisting Systems for Wood Structures (DES130_AWC08)

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AMERICAN FOREST & PAPER ASSOCIATION

American Wood Council

Engineered and Traditional Wood Products

Lets discuss IRCs wall requirements and talk at length about bracing.

Although the program will talk in some depth about the IRCs prescriptive

requirements, it will also address general conditions which necessitate

bracing.

1Copyright 2004 American Forest & Paper Association.All rights reserved.


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This eCourse explains different construction types and the behavior of small

structures and structural elements under gravity, seismic and wind forces.

Principles and typology of lateral load resistance systems are discussed

including prescriptive braced wall lines as addressed by the IBC/IRC. An

introduction to engineered shear wall design, location, and inspection points

is offered. Throughout, this eCourse demonstrates how AF&PAs new WoodFrame Construction Manual addresses these topics for one- and two-family

.

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This eCourse seeks to provide some answers, along with background on the

reasons why. Listed here are the topics that well explore in this eCourse,

beginning with fundamentals.

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Why does the code require bracing?

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Loads on the framing tend to make the framing move. Even vertical loads

such as snow loads can make the framing try to rack.

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Most loads on a buildin are a lied b vertical ressure or

gravity. Roof framing bears on wall framing which bears on the

foundation.

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Dead loads is another term used often in the code and is typically used to

indicated the weight of the building materials themselves. However, it can

have wider meaning with static loads.

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Live loads another common term are loads resulting from the use of the

building. This includes people in the building, furniture, and other non-fixed

features.

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It snows in various regions of the country, and 30 to 70 psf ground snow

load provisions are included in the 2001 WFCM (more about that document

later). Snow load span tables automatically reflect the consideration of

unbalanced snow loads.

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Lateral loads will also be a lied to a structure. The most

common is wind. Seismic forces will also occur in many parts of

the country.

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The Federal Emer ence Mana ement Association FEMA

developed these charts for wind and seismic zones in the United

States (although theyre similar to ASCE 7 maps they arent).

Notice on this wind chart that while the coastal areas around the

Gulf of Mexico and Atlantic Ocean are in a high wind zone, so ismost of the central part of the country. This apparently is a

which can generate high straight line winds and tornados.

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Wind loads can be resisted b desi n. Althou h most tornados

cant be resisted by an economically feasible design, proper

bracing can lessen damage to structures that see a near-miss

from a tornado.

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The Fu ita scale is used to rate the intensit of a tornado b

examining the damage caused by the tornado after it has passed

over man-made structures. F-0 has light damage while F-5 has

incredible damage. Over 80% of all tornados are classified at F-3

or below.

The reason for this slide is to illustrate that different tornados pack

different punches. The majority of tornados are not the incredible

ones. A word of caution, however, these lower strength tornados

cancause horrific damage. We dont intend to imply otherwise.

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This ra hic shows the im act of the tornado. Homes in the

direct path were completely destroyed while those a block or two

away received a variety of damage. The home about three

blocks away received minimal damage.

No one will argue that it doesnt matter how we build a structurethat is going to receive a direct hit from an F5 tornado. Its the

buildings on the periphery of such an event and buildings

involved in less severe storms that will perform differently

depending on the construction.

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This ra hic shows the im act of the tornado. Homes in the

direct path were completely destroyed while those a block or two

away received a variety of damage. The home about three

blocks away received minimal damage.

No one will argue that it doesnt matter how we build a structurethat is going to receive a direct hit from an F5 tornado. Its the

buildings on the periphery of such an event and buildings

involved in less severe storms that will perform differently

depending on the construction.

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One other feature of high wind events is wind-borne debris that driven by the

wind can cause moderate to severe impact damage. During the La Plata,

MD, Tornado, blankets and sleeping bags were found tossed into trees and

power poles. In this example, a 3x6x 8 long lumber section skewered the

second story exterior wall of a building, penetrating it by several feet.

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Here is a hoto of the ath of an F5 tornado that went throu h

Oklahoma in 1999. Many homes close to the tornado path

survived this tornado. Was this by chance or luck, or did the

method of construction have something to do with it?

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Proper design of wood structures to resist high wind loads requires the

correct use of wind load provisions and member design properties. A

thorough understanding of the interaction between wind loads and material

properties is important in the design process.

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Wind-structure interaction is highly complex. Wind can induce a variety of

structural responses as a whole building, and on individual components and

assemblies, as seen here. Each of these responses needs to be checked

for structural integrity as part of the wind design process.

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When wind is a lied to one side of a structure, it wants to ush

the end wall and roof in the prevailing direction. In addition, the

wind wants to pull the opposite end wall. While this is occurring,

the foundation acts to hold back the walls. Hence, thewall is

subjected to most of the force.

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Seismic loads arise from ground shaking which can be a result of many

causes. Earthquakes by far are the most serious and unpredictable seismic

load type. Other more predictable seismic loads arise from human-induced

activities.

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The primary feature of seismic motion is that its magnitude varies with time.

The time variation can be very short, such as a sharp jolt, or longer, such as

a slow rumble. Moreover, the motion direction is typically random and

constantly changing. Such a behavior can be described in terms of a wave.

Using recorded seismic data, we can describe a seismic load mathematically

in terms of a wave function of distance and time.

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Waves have three primary characteristics:

amplitude (the magnitude of the wave),

frequency (the number of complete wave cycles per second) or inversely its

period (the number of seconds per complete wave cycle), and

duration (the time lapse of the wave).

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Damping describes the decay rate of the wave amplitude as the wave dies

out. Friction in the wave generating system is an example that causes

waves to damp.

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With respect to dynamic response, buildings offer three primary

characteristics:

mass of the building or sub-assemblies,

stiffness of the building structural system,

and damping inherent in the building construction.

These can be simply modeled as the lollypop shown here. If the stick of

e o ypop s su c en y n ow s ness , an e mass s pu e ac

and released, the mass will swing back and forth in free motion. The free

sway motion can be described by the mass displacement wave shown here,

with measurable frequency. This simple sway mode is known as the natural

frequency of vibration. Mathematically, the sway motion equation takes the

form of a second order differential equation with respect to time.

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In the motion equation, all the components of the dynamic structural

behavior are evident. The equation here is written in terms of linear

displacement, x, although angular displacement terms (not shown) and other

directional displacements may be present. Solution techniques for this

equation exist mathematically.

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Solution of the motion equation can lead to a modal result: a series of

frequencies at which the structure will freely vibrate if disturbed. An example

of this solution can be heard when a guitarist uses fret harmonics (octave

pitches) to tune a guitar. In a building, the sway shape takes different forms

that correspond to the modal frequencies in the solution. Thus, a structure

can have a numberof sway modes with associated frequencies of vibration.

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Lets put the whole seismic problem together now. Ground shaking occurs

with a certain acceleration, a, moving the soil under the building. As the soil

moves, the building mass wants to stay put due to its inertia, putting a force

on the structure equal to the mass times the exciting acceleration.

Eventually the mass moves, lagging the exciting acceleration, causing

further inertial forces to develop on the structure. This gets even moreproblematic when the exciting acceleration changes direction, as the mass

movement. This is sometimes referred to as the whipping force.

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If the exciting wave characteristics match any of the buildings modal wave

characteristics, then resonance results when the exciting and response

systems vibrate in unison. Resonance is very dangerous since the response

system normally self-destructs due to its inability to cope materially with the

exciting wave. Hence, it is very desirable from a building design perspective

to separate building modal response frequencies from any potential excitingfrequencies.

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WFCM Chapter 2 Engineering provisions are based on the ASCE 7-98

Equivalent Lateral Force procedure. Building masses/weights are calculated

and collected at floor plane levels. The seismic event base shear is

calculated from the seismic loads and distributed on the basis of weights at

each story as story forces. Finally, story shears are determined for each

floor by adding all the story forces above the floor of interest. The storyshears are the forces applied to the top of the lateral force resisting system

.

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Buildings move under dynamic conditions. Two principle movements are:

racking and twist.

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Building forms impact how lateral forces get transferred into the vertical

supporting elements. Heres an example. A floor has a center of mass

located somewhere in it. The structural system below provides a torsional

stiffness that can also be centered somewhere within the floor plane. If the

stiffness and mass centers coincide, then the building will simply rack in the

direction of the applied lateral load. If however, the mass and stiffnesscenters are displaced, the building frame will twist. The greater the

, .

the top of the shearwalls can also become very large. Thus, good design for

lateral performance would suggest that centers of mass and stiffness be

kept in as close proximity to each other as possible.

This subject is important for a rigid analysis where the stiffnesses of the

.

analysis: flexible components that lend to a tributary area approach for the

loads.

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There is another way - a technique that dates back to early human

inhabitation some 10,000 years ago. Archaeological findings prove the

theory of light and strong in known seismically active areas of the earth. The

theory holds that humans discovered early that heavy things fall down easily

when disturbed with catastrophic results. Light things are not disturbed

nearly as easily, and are much easier to support and be made strong. Thus,the simple tent has become a common domestic structure to many peoples

, .

frame structures tend to fit this philosophy, mainly because of woods very

high strength-to-weight ratio.

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Now lets look at some strategies used in buildings to resist lateral forces,

primarily through wall framing methods.

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Heres a summary of what weve discussed so far.

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Without being braced in some fashion, wood frame walls tend to rack in

response to loads.

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Bracing whether its to resist the day-to-day loads on the buildings and the

typical storms or whether its to resist high-wind or high-seismic events is

critical to the performance of the building.

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Here's an example of what can happen when the lower floor of a building

doesn't have adequate bracing.

This was a building that had 9 condo units, some of them 3-stories tall,

which collapsed in 40 - 50 mph wind. A structural collapse while underconstruction isn't unusual because not all of the required bracing may be in

place when a storm strikes. But as you can see in this case, the exterior

walls were being bricked and framing was almost complete. If you look at the

unit on the left, you will see that the lower floor had mostly doors and

windows and very little braced wall area, indicating poor design.

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There are fundamentally three ways to stop a frame from racking. Well talk

in some detail about the application of these methodologies, but all of the

bracing materials and systems involve some version of what you see here.

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Lets talk in some detail about the methods of providing bracing, starting with

the use of triangles.

Diagonal tension ties create a triangular geometry within the frame that in

itself, is a stiffening element. Compression ties are rarely effective, if at all.Diagonal board sheathing, however, works in this mode.

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This is one of the simplest ways of providing lateral resistance to a wall

assembly. However, let-in braces require a perfect and well connected fit in

order to work properly, which is often difficult to achieve. And, they cannot

provide the same capacity as a properly constructed wood panel shear wall.

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Proper installation and connections are key to making this method work.

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A very efficient way to brace walls, and one that was common years ago, is

to sheath the wall in diagonally oriented boards.

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Triangular geometry is used profusely in the truss industry for the trusses

themselves, as well as the bracing of them in the context of an entire structural roof

or floor system. For peaked roofs, trusses are braced in three planes.

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This is a photo of a demonstration assembly that shows the different kinds of

bracing. The ground bracing is in dark blue. Notice how the ground bracing is

braced with triangles. Also notice how the lateral bracing on the top chords (in

orange) line up with the ground bracing making the truss more stable.

Photo showing continuous lateral bracing (CLB) and the diagonal bracing

needed to restrain the CLB.

Diagonals brace the bracing. It shows rows of lateral bracing being tied with

agona rac ng. ose agona s s ou not e more t an to eet apart.

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Temporary bracing is one of the most critical issues facing the wood truss

industry when it comes to construction safety. This Truss Technology in

Building shows how critical diagonal bracing is when using short spacer

pieces for lateral bracing as is typical on construction sites today.

The WTCA Warning Poster completes the educational information that is

needed at the jobsite to install and brace trusses safely. For more

information on bracing contact WTCA or visit the WTCA website at

www.woodtruss.com.

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For long term roof system performance the proper installation of permanent

bracing is a necessity. This TTB defines the rule of thumb approach for

trusses. Where possible, however, the Building Designer should provide a

permanent bracing plan.

WTCA also has a Commentary For Permanent Bracing of Metal Plate

Connected Wood Trusses that goes into more detail. For more information

contact WTCA or visit our web site at www.woodtruss.com

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Shearwalls are a vertical building element that can resist lateral forces

applied at the top of the wall. In a wood shearwall, the panel perimeter nails

provide the bulk of the racking resistance through wood bearing and nail

deformation when the lateral external force is applied. Horizontal wall sliding

is resisted by nailing or other anchorage installed along the bottom of the

shearwall sufficient to resist the external lateral force.

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In order to make this concept work, panels must have a height-to-width

aspect ratio of less than 3.5 to 1. This ratio is sufficient to develop racking

action in the shearwall panel. Aspect ratios greater than this produce

cantilever beam action - a completely different behavior that is much less

effective in resisting lateral forces. The concept of aspect ratios is

incorporated into prescriptive bracing requirements, but isnt specificallyaddressed. It is, however, that basis for limits to minimum widths of various

.

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Although they arent part of the wall bracing systems, roofs and floors take

the loads from the walls and transfer them to the foundation. Aspect ratios

for roof and floors arent addressed prescriptively, but in engineered design

these elements, called diaphragms, have length-to-width aspect ratio limits

as wall as limits on openings.

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In its prescriptive provisions the IRC refers to braced walls as braced wall

lines. In engineered design, however, the bracing is provided by shearwalls.

Shearwalls feature special nailing and hold-down connections designed to

resist applied lateral loads in shear and overturning. Minimum wall aspect

ratios apply in order to develop shearwall action as opposed to cantilever

beam action when the wall panel aspect ratios become very slim. Typically,the closer to the minimum aspect ratio for a shearwall, the more dense the

. ,

the most effective in resolving the transferred applied forces.

A more convenient method is the use of shearwall systems: panels, or

entire walls. Shearwalls feature special nailing and hold-down connections

designed to resist applied lateral loads in shear and overturning. Minimum

cantilever beam action when the wall panel aspect ratios become very slim.

Typically, the closer to the minimum aspect ratio for a shearwall, the moredense the nail perimeter nail spacing. In shearwalls, it is the perimeter

nailing that is the most effective in resolving the transferred applied forces.

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Diaphragms are usually horizontal surfaces that resist in-plane shear forces.

Nailing is more dense where the shears are highest typically in the panels

around the diaphragm perimeter.

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In the second general bracing methodology, the perimeter-nailed panel

resists racking through the resisting action of the perimeter nails to the

applied racking moment on the panel. Nailed horizontal boards with at least

2 nails on the same stud has the same effect, but to a lesser degree. Here

the nails do most of the work.

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A third method absorbs the racking moment directly in the rigid joints in the

corners. This is called a moment frame since the rigid corners induce

bending moments in all the members near the rigid connections, so

indirectly, the frame members also resist the racking forces through flexure.

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Proprietary systems such as APA Sturd-I-Frame try to utilize normalconstruct on a coup e o anc or o ts an ntegrat ng t e ea er ansheathing with nails. Notice the attention given to making the corners rigid.

Construction of these types of assemblies requires careful attention to

details.

Proprietary systems such as APA Sturd-I-Frame try toutilize normal construction a couple of anchor bolts andintegrating the header and sheathing with nails todevelo the ri id frame oint.

Note a few things that are important to this detail..

If the overall wall height is more than 8 feet (say youbuild a cripple wall on top of the header), the panel joint

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s ou a w n ee o e m - e g o e raceportion of the wall.


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Remember this house? Notice the failure of the garage door header at the

corners. In dwelling construction, this type of wall design an opening with

small braced wall sections on either side is ideal for the application of a

moment frame.

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Braced wall lines are employed on the building-level scale to develop lateral

resistance in two orthogonal directions for the structure.

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Now lets look at the prescriptive method in which the IRC addresses wall

bracing. The code approaches the subject by specifying where the bracing

is to be placed, what materials are acceptable, and what quantities of

bracing materials are needed.

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Braced wall lines are walls made up of a series of unbraced sections and

sections of walls that are braced with acceptable materials in the required

amount. These braced sections are called braced wall panels.

The braced wall lines are required to be placed in both directions of the floorplan and are required in each story.

Often the exterior walls will provide the required braced wall lines. However,

when the distance between exterior walls is too large, an interior wall is

required to be a braced wall line. The IBC is specific about the maximum

distance between braced wall lines being 35 feet. However, for some

reason the 2000 IRC is vague. The only mention on spacing is in Section

R602.10.11, which addresses spacing in high seismic areas. In the 03

edition, the IBCs 35 feet provision has been added.

Braced wall lines are permitted to have an out-of-plane offset of no more

than 4 feet in one direction, with a provision saying that total offset can be no

more than 8 feet. This would allow a wall to have a 4 foot offset in each

direction.

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The concept in the IRC is to break the structure of the building into boxes

with a limited aspect ratio.

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This is a very rough schematic illustrating the general code provisions for the braced wall

ne concept. exter or wa s must e race wa nes. nter or race wa nes are

required if the spacing between exterior walls is excessive (greater than 35 in the 2000 IBC

and in the 2003 IRC).

Individual portions of wall that contain bracing material are called braced wall panels.

An offset in the overall braced wall line of 4 is permitted. As illustrated in the lower left

portion of the floor plan, the code will allow an offset of up to 4 in each direction, meaning

that the bottom exterior wall could have been offset as shown by the dotted line.

Openings are permitted at the corners of the braced wall line as shown on the exterior wall

on the left. There is some confusion in the code, however, on this account. Section R

602.10.1 permits the first braced wall panel to be 12-1/2 ft from the corner. Then it says that

when the bracing begins more than 12 from the end of the wall a designed collector is

requ re . t s unc ear w et er t s was nten e to requ re a co ector o some sort w at s

termed a drag strut in seismic design) if the braced panel is placed in that permitted 6

beyond 12-0, or whether the intent is for a collector to be required in all instances in whichthe first braced panel isnt at the end of the wall.

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The code lists 8 general materials that are acceptable as bracing, and we'll

talk more about those in a moment. Not all of those materials, however, are

acceptable as bracing materials in all instances.

The seismic category or wind speed zone, as well as which story is beingbraced, will determine whether a specific material can be used and how

much of the wall must be braced using that material.

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Here are the 8 general bracing materials. The numbers that you see here

are the numbers that you'll see on a table that we'll talk about in a moment.

Well also talk in some detail about the various materials.

These are the materials that the code accepts outright. That's not to saythat there aren't other materials that will provide adequate wall bracing.

However, any other material must be addressed under the alternate

methods and materials provisions of the code.

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This is a relatively formal way to say that were not sure exactly what

resistance to lateral loads are being provided by prescriptive bracing. We

know from experience that it works under the limitations of conventional

construction. However, since the wall isnt formally designed and it lacks

elements of a shearwall, such as connections to the foundation or floor, its

hard to quantify the resistance to any exact degree.

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In a formal shearwall design, we can quantify the shear resistance in bracing

material; in fact, the code provides those numbers for everything but let-in

bracing. But because the overall resistance to racking in conventional

construction isnt completely understood, we dont know exactly what shear

resistance is being provided by the bracing material itself. Here are some

estimates of the shear strength of the 8 allowed bracing materials appliedaccording to the IRC. The widely varying numbers explain why different

.

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Now, lets talk about the details of the various methods of bracin .

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One of the bracing methods accepted in the code is 1x4 let-in bracing such

as you see here.

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R602.10.3(1) calls for the let-in bracing to be no less than 45 degrees from

the horizontal and no more than 60. It also calls for the 1x4 to be let-in to

the top and bottom plates as well as the studs.

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But let-in bracing will be effective only if it's installed properly. Here you see

an example of poor installation. The notches in the studs are so wide that

the 1x4 isn't being held tightly. If the wall tries to move the wide notches are

going to permit some racking before the 1x4 comes into contact with the

edge of the notches.

And it also appears that some of the nails may not be properly driven into

the studs. Notice that the nail head in the yellow circle appears to fall right

on the outside edge of the stud, meaning that probably only one of the nails

is providing any resistance.

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As mentioned before, diagonal boards provide a very efficient bracing

material, but that method has fallen out of favor because of the availability of

panel products.

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Wood structural panels are very desirable bracing materials, particularly

when used in lower floors of multi-story buildings or in buildings subject to

high lateral loads. Additionally, the use of these panels to completely sheath

a building will offset some other problems that well talk about in a moment.

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Keep in mind that structural fiberboard and particleboard must be

manufactured in accordance with standards referenced in the code. While

they might look like the materials used in cheap, short lived furniture,

compliance with those standards assures a long lasting structural product.

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Gypsum wallboard is acceptable as a bracing material, but because its

brittle and easily crushed, its values are limited and longer lengths of it are

required to provide racking resistance.

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Keep in mind that the prescriptively permitted stucco cited in code is

traditional portland cement stucco and not EIFS or other stucco-looking

material.

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Because it has a rather soft surface, its important to prevent overdriving of

fasteners. If over driven, two problems are created:

1. If driven far enough, the amount of the nail shaft that bears on the wood

fiber to resist shear is less, lowering the shear resistance value.2. Broken surface permits moisture get into the panel around the fastener,

.

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Table 602.10 in the IRC specifies the amount of bracing material that is

required and where the bracing is to be applied.

Here is the first of the table. Notice that its predicated on the SDC and/or

wind speed for the location in question and the story to be braced. Then the

acceptable types of bracing, identified by the numbers on the previous slide,

are listed. The final column talks about the amount of bracing panels

required and the location of the panels.

The lower seismic zones A, B & C -- require the least amount of bracing.

Zones D1 and D2 require much more because of the intensity of seismic

forces. Where wind speeds exceed 110 mph, engineered shear walls are

required.

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The site of the building must be reviewed for applicable Seismic Design

Category and Design Wind Speed.

The lower seismic zones A, B & C -- require the least amount of bracing.

Zones D1 and D2 require much more because of the intensity of seismicforces. Where wind speeds exceed 110 mph, engineered shear walls are

required.

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Here we see the acceptable types of bracing which are based on which story

is being braced.

Notice that all 8 bracing methods are acceptable in 2-story buildings and in

the top 2 stories of a 3-story building. But the let-in 1x4 is not acceptable inthe bottom story of a 3-story building.

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The table specifies the amount of bracing material that is required and where

the bracing is to be applied. Notice that for lower stories the overall amount

of bracing varies with the type of bracing material and with loads carried from

floors and walls above.

The higher floors require less bracing because they carry less load.

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The table contains a provision that is in conflict with Section R602.10. That

section permits the first braced panel to start at a point 12 (or 12-1/2) ft from

the corner. The table says that the bracing is required at the end of the wall.

This appears to be a correlation error. The intent of the IRC drafting

committee was to permit the bracing panel to be placed some distance from

the end of the wall. This table was, in all likelihood, borrowed from someother source and this at the corner statement was erroneously retained.

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As previously stated, alternate bracing methods and materials may be

accepted. However, its important that their acceptance be based on the

knowledge that they will perform adequately when loaded.

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The point to make here is that the provisions of Table R602.10.3 and

Section R602.20-4 -- which addresses minimum width of bracing panels -- is

going to mean that often narrow wall sections won't be acceptable as braced

panels in braced wall lines.

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The code stipulates a minimum length of braced wall panel for most of the

materials. This means that the short sections of walls, even if theyre wood

structural panels (Method 3), cant be counted toward the required amount of

bracing. This is an example of a built-in aspect ratio limitation similar to the

1:3.5 aspect ratio we talked about for engineered shearwalls.

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However, the IRC permits narrow all sections if the building is sheathed

overall with wood structural panels.

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Heres a summary of that provision. This will permit narrower braced wall

panels, but keep in mind that there will still be some limitation on how narrow

they can be.

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While we all know intuitivel that a full sheathed house

outperforms one with only minimal bracing units, we also know it

by experience. The house fully-sheathed with plywood in the

foreground performs much differently than this foam sheathed

homes in this 2003 high-wind event. This type of performancedifference was also confirmed by NAHB testing in 1998.

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Do the braced wall line provisions apply to garage walls? Yes. If the narrow

walls on the sides of the garage opening cant comply with the minimum

widths required for the bracing material chosen even if the building is fully

sheathed there are options permitted by the code. However, they are very

restrictive.

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Heres a summary of the alternate braced wall system provisions of the code

that were intended to be used in the narrow sections of wall on either side of

a garage opening.

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APA The Engineered Wood Association recently had a code change

proposal approved that will revise the alternate braced panel section of the

code to permit wall construction similar to what you see here. This will

permit a more user-friendly and less expensive alternative in lieu of whats

currently in the code.

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Although the prescriptive bracing requirements in the IRC address

construction in some high-seismic areas, wind is limited to areas with a

design wind speed of less than 110 mph.

If the building is located in a higher wind area it must be designed accordingto ASCE 7, must comply with the provisions of SSTD 10, or it must comply

with the engineering-base prescriptive provisions of AF&PAs Wood Frame

Construction Manual.

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Shear walls are engineered wall bracing that are designed to withstand high

forces. Often these are used with design professional input.

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In the design of irregular structures, guidance for designing irregular

structures as separate structures or inscribed is provided. Typically, the

inscribed technique can only be used for wind design. Splitting into separate

structures can be used for either wind or seismic design, and often is the

preferred method.

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Now we come to Section 2305, Lateral Force Resisting System.

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Section 2305.1 contains these provisions.

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The use of diaphragms to withstand lateral forces is an integral part of both

ASD and LRFD design methodologies. Section 2305.2 contains a deflection

formula that can be used for diaphragms.

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Similarly, a deflection formula for shear walls, and other provisions for shear

walls, are found in 2305.3.

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This is one of the helpful diagrams in the chapter, which helps to define the

height and width of shear walls and shear wall segments in walls with

openings. These definitions are used in the determination of the aspect

ratios for the shear wall

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A shear wall (wall brace) is essentially a deep, thin cantilevered beam

projecting from the foundation that is subjected to one or more lateral forces,

such as those due to wind or seismic activity. Listed are the primary

components of a complete shear wall design.

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The code provides two alternative methods for the design of shear walls.

There is the traditional method, in which force transfer around wall openings

is actually designed.

The other alternative is the perforated shear wall method, which is the resultof recent research about the shear capacity of shear walls where there is no

specific provision for transfer of force around the openings

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Reassessment of the inherent shear capacity of shear walls under wind

loads led to a 40% increase in shear capacity of Wood Structural Panels

(WSP) for wind design only.

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The shear capacity of a shearwall segment sheathed on both sides to resist

wind loads only is additive.

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Perforated shear walls have reduced shear capacity from the traditional

segmented wall, but interior holddowns have been eliminated a usually

beneficial feature.

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Three sources were used in development and substantiation of perforated

shear walls, including Sugiyama, APA-The Engineered Wood Association,

Virginia Tech, and NAHB Research Center tests.

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From these tests, correlation is very good and conservative.

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Shear capacity is reduced using these effective shear capacity factors,

adopted into the Standard Building Code. Critical features in this table are

percent of full height sheathing, and maximum unrestrained opening height.

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To demonstrate the effects of each of these three design methodologies.

First look at an example of the traditional shearwall using the unadjusted

shear capacities. Note that a dozen holddowns are required along the length

of the first floor.

. ,

the wind uplift is much greater than 60% of the dead load, and will require

more than the dead load alone to offset the wind force component. For

seismic loads, the holddowns are conservatively sized in the WFCM to meet

the shear capacity of the shearwall.

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WFCM 1997 Supplement Table 3B is used to determine shearwall capacity.

Assuming SPF framing with G=0.42, 8d nails, 15/32 Structural sheathing,

and nailing of 6/12 and 4/12 along the panel edges.

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By using the perforated shearwall method the amount of sheathing has been

increased, however holddowns are only required at each end of the

shearwall.

The WFCM uses 60% of the dead load to resist wind uplift. In most cases,

the wind uplift is much greater than 60% of the dead load, and will require

more than the dead load alone to offset the wind force component. For

se sm c oa s, e o owns are conserva ve y s ze n e o mee

the shear capacity of the shearwall.

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Using the perforated shearwall method, shear capacity is reduced using

effective shear capacity factors. Assuming an 8 wall height, window

openings are H/2 or 4 and door openings are 5H/6 or 68. Interpolation is

permitted based on percent full-height sheathing in the wall.

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Here are the wall assembly assumptions used for the development of the

preceding methodology. A cooler nail is also known as a drywall nail.

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Back to the 2003 Missouri storms which fell into the F3 or less

category, our damage assessment team, which was made up of

members of the American Association of Wind Engineers,

estimated that 50% of the failures initiatedwith poor lateral

capacity of walls enclosing two-car garages, such as this one.These are the types of storms where how a home is built can

make a big difference.

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so dont forget the holdowns. Youll find a handy table in 2001 WFCM

Table 3.17F.

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Heres a handy list for referencing design aids for anchorage against wall

sliding and/or uplift. Roof/truss anchorage are also included in the Table 3.x

list, among other assembly connections. Many of these tables will give a

connector capacity, or a connector spacing as a result.

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Hold-downs are required to prevent a wall panel from overturning. Hold-

downs may also be used elsewhere to prevent uplift, and to tie the structure

load path together to the foundation. Typical calculations are provided for

hold-down connections in AF&PAs LRFD Manual, Example 7.7-1.2.

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The conventional construction bracin re uirements of the code

are required to handle lateral loads as determined by the

buildings site location.

Handling lateral loads in California is a whole different ballgame,

as can be seen from this residential construction detail. We havelateral loads and the code rightly stipulates that our structures

need to handle them appropriately.

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while these apply to intermediate stories.

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.and when the forces get large, the connecting hardware gets more

interesting.

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Here is a glulam beam tension connection good for up to 75 kips.

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and here are the referenced standards that will apply.

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The IBC and IRC both reference engineered construction. The IRC

references the WFCM, which is engineered construction. The IBC also

references engineered construction, but much of the prescriptive provisions

of the IBC are still based on conventional construction. Some of the tables in

both the IBC and IRC have an engineering base. To clarify the scope of the

two documents: IRC = detached and attached one- and two-family; IBC =multifamily, although some designers have used the lateral bracing

, .

the 40 psf floor loading.

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To aid designers, the WFCM 2001 Workbook (free from www.awc.org) has

been developed. It is a real design example using a real house in a real

location and the workbook provides tabulated calculations with complete

references to WFCM and blank worksheets for future designs of your own.

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lateral load resisting systems for wood structures (des130_awc08)

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